JPS5848387A - Induction heating cooking device - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an induction heating cooker, and particularly to an induction heating cooker that uses a second transformer type excitation oscillator as a drive source of an inverter and has a protection function for a non-magnetic metal pot.

Conventionally, in this planter, when the inverter's pot made of magnetized metal is heated, the impedance of the induction heating coil and the resonant capacitor is small (1), and an excessive current flows through the switching elements that make up the inverter. This had the disadvantage of causing deterioration and destruction of the element.

The present invention has been made to eliminate such drawbacks, and will be described in detail below based on embodiment F.

In Figure 1, fi+ is an AC power supply, (21 is a choke coil, (31 is a rectifier (B) path, (C is a smoothing capacitor, and (Q is two transistors connected in series. , works as a unidirectional switching element. (4)
is a series resonant circuit consisting of a resonant capacitor (C) and an induction heating coil ILI connected in parallel to the transistor (Ql);
This is a diode connected in antiparallel to the The above transistor (Q') (Q, series resonant circuit +41 E and diode (Dz), H and inverter circuit (
5) is constructed. (CT moon, induction heating coil TIJ
(6) is a current transformer that detects the load current 1L flowing through the transistor (Q),
A control circuit that outputs signals b, o, and d drives and controls an inverter (51). Here, signals a and c are output from transistors (Q
The signals S and d are pulse signals with opposite phases applied to the bases of each transistor (QS(Q).
It adjusts the potential to the base of the transistor (Ql) to ensure its operation.The collector potential of the transistor (Ql) is set to vC.
When the emitter potential of the transistor (Q) is set to the ground potential, the series resonant circuit (4) is alternately connected between the potential Vcc and the ground by turning on and off the transistor (Ql) (Q). When the resonance frequency of the resonant circuit (41 is high), the impedance becomes zero, the waveform of the current flowing through the induction heating coil ILJ is a sine wave, and the transistor that handles the current changes every half cycle.Such an inverter circuit (51) The oscillation frequency of about 20
A series resonant circuit (41) is adjusted so that the heating frequency is kHz, and a heating pot (not shown) made of iron-based metal is brought close to the induction heating coil ILI to heat it by induction.

The control circuit (6) is composed of a two-transformer self-excited oscillator 8
Then, the power supply voltage VaO is applied via a diode (D). This voltage 1/cc draws a pulsating current waveform that is hardly smoothed because the capacitance of the smoothing capacitor (C) is small for power factor correction. (T is the first transformer, two transistors with the same characteristics (Q) are connected complementary to each other, and each collector winding (nxJ constitutes its primary winding). The emitters of each transistor (Q) are connected in common and connected to the intermediate point of the primary winding via a smoothing capacitor (C1), and at the same time a diode (D
It is connected to the power supply voltage Vco terminal via the cable. (T2)
The base winding (nj) of the second transformer C1 transistor (Q) (nj) constitutes the secondary winding.

(H+P is a transistor (Q) is a resistor inserted between the base and emitter of Q, (Ha) is a starting resistance, (D
＜）(Cs Month Here, the base winding of 1i21-lance (T2) is connected in parallel with the diode and capacitor (n
A diode (D4) is used to prevent reverse current flow, and a capacitor (C4) is used to increase the switching speed. .

(n is the positive feedback line of the first transformer (T), and its output is connected to the primary winding (nj) of the second transformer (T) via the resistor (I). The second transformer (・r) is a so-called saturation transformer that is used up to its saturation region. (nsu(n@) is the first transformer←r
The secondary windings of the transistors ((15) and (Q) are respectively provided corresponding to the collector windings (n of , a resistor (H5), and a capacitor (C), and are subjected to overlap prevention processing and then output as signals a, b, c, and d.

These control signals a, b, c, and d are applied to each base and emitter base of the switching transistor (Q) described above to control the oscillation of the inverter circuit (5).

t7+ is a rectifier circuit that rectifies the voltage induced in the positive feedback winding (n) of the first transformer (T-+); “r
In the control winding, a resistor (H) and a transistor (Q) are provided in the middle.
A resistor ('')+ and a variable resistor (VRJ) are connected between the bases.

(8; is an overcurrent detection circuit that detects the excessive current flowing through the transistor (Qll) when an overloaded pot made of one non-magnetic metal such as copper or aluminum is placed on the induction heating coil tLl. Yes, (9) is a current transformer (CT).The rectifier circuit to which the detection current is input, (
H is the resistance that converts the detection current to voltage, (zD) (D
It is a Zener diode and a diode connected in series, (Q is a transistor (a transistor connected in parallel between the emitter and collector of the noise), and its base is:
It is connected to the diode (1) IJ and serves as an impedance element (C7) is a capacitor that delays the off timing of the transistor (Q).

Next, the operation will be explained. When the voltage VaO is applied to the control circuit (6), if the smoothing capacitor (C) does not smooth the transistor (Q) and it becomes conductive, a current begins to flow between its collector and emitter, and the collector winding (n An induced voltage is generated across the positive feedback winding (n) which is coupled to the second transformer (!S0), and the base winding (n of the transistor CQm via the Further, an induced voltage is produced in the transistor ( Due to the primary inductance of T, it increases linearly and becomes saturated.This causes the second transformer (Tt
) suddenly increases, the voltage drop across the resistor (H) increases, which causes the voltage across the primary winding (N) of the second transformer (Teri to decrease, The feedback voltage decreases.Then, the base of the transistor (q) is
A voltage is applied to the capacitor (C) with the polarity shown in the diagram,
Transistor (Q is cut off, transistor (
Q4) starts to be energized. Then, a feedback effect occurs in the opposite direction to the previous one, turning off the transistor (Q) and turning on the transistor (Q).The primary current of the second transistor (T) is reversed, and the transistor (Q) Since it is electromagnetically coupled to the transformer n7 in the same way as when it is on, an output is obtained depending on whether the transistor (Q) is on or off, and when the transistor (Q is on), On signals a and b are obtained, and signals o and d are off signals, and when the transistor (Q) is on, signals a and b are off signals, and signals o and d are on signals.

When signals a, b and signals o, d are expressed as waveform numbers,
It can be drawn as shown in Figure 2.

Next, the frequency variable operation will be explained. Due to the voltage induced in the positive feedback winding of the first transformer, when a magnetic current bias is added to the second transformer, the magnetic current increases and reaches saturation earlier. From this, the oscillation frequency can be changed by changing the bias voltage value.The bias voltage is changed by changing the resistance value of the variable resistor (VR).

In other words, the collector-emitter potential of a transistor (Q) is held constant, and its level is controlled by a variable resistor (
Since it can be changed depending on the VR month, the control winding (nz
The voltage appearing on the opposite side is the value obtained by subtracting the collector-emitter potential of this transistor (Q). Therefore,
By adjusting this potential, the control winding (nl
As a result, the potential applied to the second transformer (T
The magnetic current bias will be changed.

Figure 5 shows the control winding (nl) of the second transformer (T).
The specific winding structure of the primary winding (n) and the secondary winding (n4) is shown, and the control winding (nl) (nlOJ, 2
Each of the toroidal cores 11 (manufactured by L. Co., Ltd.) is wound independently and connected so that the current directions are opposite to each other.

The primary and secondary windings (n.
1- is wound M in the same direction. This can prevent AC voltage from being induced in the control winding.

FIG. 4 is a diagram showing the operating state of the inverter circuit +5) using the two-transformer self-excited oscillator configured as described above, in which the vertical axis represents the oscillation period and the horizontal axis represents the power supply voltage v3. As shown in the figure, when the value of the variable resistor (VRJ) is divided into five stages (large, medium and small) and oscillated, each has a period of approximately 50/As (frequency: 3
3KHz), approximately 40μs (frequency 25KHt),
Approximately 507A8<20 KHz in terms of frequency), and linear frequency control is possible in the range from 20 KHz to about 33 KHz.

In the above example, the current flowing through the control winding (n+) was controlled by a resistor (H), a variable resistor (VR), and a transistor (Q). It is also possible to use variable resistors.

Next, the operation of the overcurrent detection circuit (8) will be explained using FIG. Now, suppose that a suitable load such as copper steel is placed on the induction heating coil (IL) instead of a hot pot.

Load current 1L in the case of a copper pot and load current 1L in the case of a copper pot.
The respective waveforms of L are shown in C and O. In addition, in the waveform DI, the solid line indicates the transistor (Q) and diode (D).
The dashed line shows the current flowing through the transistor (Q) and the diode (D).
(If the voltage of the Zener diode (ZD) and the diode (D) is set to a voltage corresponding to the rated current BF of I, for example, about 6OA, when this voltage is exceeded, the transistor (Q6) Therefore, the bias current in the control winding ('?) Cn+o) increases and the second
The magnetic saturation of the transformer (T) occurs quickly, and the oscillation frequency increases.In this way, when overload heating occurs, the switching transistor (QS CQ2) may flow. The current waveform is shown, and its frequency is about 55 to 4QKHz compared to the steady state [2oKHz for copper pots or aluminum pots.

As explained above, the induction heating cooker of the present invention uses a two-transformer self-excited oscillator as the control circuit of the inverter,
Since it is equipped with a detection circuit that detects overload made of non-magnetic metal, when the overload detection function is activated, it automatically increases the oscillation frequency of the self-excited oscillator.
A current exceeding the rated current will not flow through the switching transistors constituting the inverter, and the switching elements can be protected.

[Brief explanation of drawings]

! 1! Figure 1 is a circuit diagram of an embodiment of the present invention, Figures 2 and 5 are operating waveform diagrams, Figure 6 is a winding configuration diagram of the second transformer,
FIG. 4 is an oscillation period-power supply voltage characteristic diagram. (51... Inverter, +61... Control circuit, (8
)...Overcurrent circuit.

Claims (1)

[Claims] 1. A series resonant circuit consisting of two unidirectional switching elements connected in series between the DC power supplies, a resonant capacitor and an induction heating coil connected in parallel to one of the switching elements, and antiparallel to each of the above switching elements. In the induction heating cooker including a connected diode, a control circuit that alternately operates the two switching elements, and an overcurrent detection circuit that detects an excessive current flowing through the Koki induction heating coil, the control circuit includes a first transformer. , and a second transformer consisting of a saturated transformer, 9711. a positive feedback winding provided between the second transformer and a control winding for controlling magnetic saturation of the second transformer;
It is composed of a second transformer-type excitation oscillator whose next winding outputs drive signals for the two switching elements, and the control current of the control winding is changed by the output of the overcurrent detection circuit. An induction heating cooker featuring: